1. Field of the Invention
The present invention relates to an engine cooling system that can be suitably used for an automobile.
2. Description of the Related Art
An engine cooling system of carried by an automobile or the like has a function of circulating engine cooling water in a radiator to cool the cooling water so that the cooling water temperature can be maintained within a predetermined range. Therefore, the temperature of each part of an engine can be maintained within such a predetermined range that the engine functions properly.
In a conventional cooling system, a thermostat provided in a cooling water path controls the amount of cooling water flowing into a radiator so that the cooling water can be maintained at a predetermined temperature. More specifically, if the temperature of cooling water discharged from an engine is relatively low as in the case of low load operation, the amount of cooling water bypassing the radiator is increased to lower the proportion of cooling water cooled by the radiator to inhibit the cooling water from being cooled excessively. If the temperature of cooling water discharged from the engine is relatively high as in the case of high load operation, the thermostat automatically opens to increase the amount of cooling water flowing into the radiator to increase the proportion of cooling water cooled by the radiator so that the cooling water can be cooled sufficiently.
Incidentally, the cooling water is required to be maintained at a relatively high temperature in order to reduce friction loss and to improve fuel economy. On the other hand, the cooling water is required to be maintained at a relatively low temperature in order to improve the volume efficiency of intake air to inhibit the engine knock and increase engine power. Thus, if the cooling water is maintained at a relatively low temperature in the case of high load operation requiring excellent engine performance and is maintained at a relatively high temperature in the case of low load operation, both the engine power and the fuel economy can be improved. In the above-described conventional cooling system, however, the cooling water must be set to a low temperature with greater emphasis being placed on the engine power or to a high temperature with greater emphasis being placed on the fuel economy. Namely, only one of the engine power and the fuel economy can be improved.
Alternatively, use of an electronically controlled thermostat may be considered for the above-described engine cooling system to control the cooling water based on instantaneous values (e.g. engine load) of ever changing engine operation.
In this case, however, the cooling water temperature cannot be controlled properly in terms of medium and long term engine operating conditions, the driver's engine operation style (e.g. the operation style with greater emphasis being placed on the engine power and the operation style with greater emphasis being placed on the fuel economy), and so forth, and therefore, both the engine power and the fuel economy cannot be improved satisfactorily.